That's kind of an odd duck of a problem, as the air will just flow to wherever there's fire......
It's possible more air is getting to the one side, but I don't think it should have that much effect ?
In trying to understand this, Questions for clarity:
You lite the whole basket, then set it down in ?
Are you lined up like the picture because a of rotisserie setup?
If you line up the long, flat side of the baskets parallel with ONE of the inlets, it will give the most equal air to each side.....
Have you tried starting out with a half basket lit (each side) & adding handfuls of coals some at a time 'till they're all burning? Or lit, a few at a time from a chimney ?
I know this takes longer, but it is possible that there's just barely over too much coal at the start & it can't get enough air to keep breathing correctly; this is the only reason I can think of that would create such a finicky situation on one side.......
I know it's the same goofy question, but dry charcoal? Shit's like a sponge as soon as the weather starts to turn; it can be as simple as an open bag & the 1st ones went on your 'dead' side.......
Below is a recent burn, the unlit ones I've just laid in there to start up an indirect for tri-tip....
The burn went all the way around, with 4~6 coals burning at a time & the only thing left is a very small unburnt piece of oak @ about 12 o'clock
This is daisy wheel vents so there's not a hell of a lot of air coming in, esp. when the ash is falling,
but if there's an accurate amount of fuel in relation to the air, everything burns
Looked back at your pic once more & are the partially burnt ones on the right just like it burned, or ones that you moved over later?
Try also lining up your lid so the vent sits in between the two baskets equally, I know this has some effect.......
Interesting problem....ya got my attention.....